This invention relates to methods of evaluating the condition of oilfield tubular goods and the extent of internal wear of subsurface pipe or casing. More specifically, this invention relates to methods for locating collars on either a worn or unworn string of drill pipe.
The determination of the location and dimensions of casing collars are important steps in procedures for determining the degree of internal wear present in a length of casing. A variety of problems may result from internal casing wear. Drill pipe collars may rub the casing, and possibly lead to a blowout in the casing if extremely worn. In a production well, badly worn casing may permit the undesirable "thiefing" of the flow to unwanted zones and thereby reduce surface production. In an injection well, worn casing may permit the injected fluid to flow to undesired formations.
Internal wear, which is the removal of metal from the inside casing wall occurs in three principal patterns: loss of metal from only part of the inside casing wall while the rest is unworn; uniform loss of metal around the entire inside casing wall circumference; and non-uniform loss of metal around the entire inside casing wall. In addition, there are two patterns associated with unworn casing--round internally with all of the original metal in place, and elliptical with all of the metal in place.
A number of surveys are available for assessing internal wear in downhole casing. One such survey provides a depth recording which responds to the circumferential average of the wall thickness of the casing metal still in place. In particular, this survey detects changes of the wall thickness and internal diameter of downhole casing. A typical inspection instrument for performing this survey is the casing inspection instrument marketed by Dresser Atlas under the name "Magnalog tool" and disclosed in the publication "Dresser Atlas Casing Evaluation Services," pages 4-5 and 62-63; said publication being herein incorporated by reference. During operation, an inspection instrument generates an alternating (AC) magnetic field of low frequency. As the tool passes through the survey interval, the field permeates the casing wall and is detected by the receiver portion of the tool. The received electromagnetic wave is both attenuated and phase shifted by the casing wall, with the degree of phase shift in relationship to the wall thickness (weight). The degree of phase shift is detected and tranmitted over the wireline for surface recording.
The phase shift curve registers the phase shift between generation of the low frequency electromagnetic wave and reception of the wave by the tool's receiver portion. The degree of phase shift increases with the increasing thickness of the casing's wall. The degree of phase shift may then be used in the calculation of the average thickness of the casing wall.
A problem arises, however, when pipe collars are present along the casing length being surveyed. The presence of a pipe collar causes a noticeable change in the phase shift curve response at the point corresponding to the location of the pipe collar which in turn results in an increase in the calculated average thickness of the casing wall at the location of the pipe collar. The casing wall thickness calculations are so distorted by the presence of the collars that accurate casing wall thickness calculations cannot be made for those sections of the casing where collars are located, and some method of editing out the inaccurate calculations of casing wall thickness becomes necessary. While the editing out of the collars after the calculation of the average wall thickness has often been suggested as a solution to the problem, this method results in a waste of valuable processing time by calculating average wall thickness for numerous sections of casing that will later be excluded. Instead, removal of the collar responses from the phase shift curve before the calculation of casing wall thickness is considered one method of maintaining the accuracy of the casing wall thickness calculation while minimizing processing time. However, the use of this approach presents additional problems when one considers that moderately to severely worn casing will often be the subject of this survey. While collars are readily recognizable on the phase shift curve of unworn casing, as being indicated by the presence of a characteristic response located at regular intervals of the casing length, the characteristic response indicating a collar becomes much more difficult to detect when the collar is flushed or welded and particularly when the casing becomes worn. Thus, a method to detect collars under these extreme casing conditions also became necessary.